Carrier aggregation is a combination of two or more cells or component carriers (CCs) operating at different frequencies in order to provide a broader transmission bandwidth for a mobile terminal. Depending upon its capabilities, a mobile terminal may simultaneously receive or transmit on one or more of the cells. The cells aggregated in accordance with carrier aggregation include a primary cell and one or more secondary cells. The primary cell is the cell that: (i) operates on a primary carrier in which the mobile terminal either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or (ii) was indicated as the primary cell in a handover procedure. Conversely, a secondary cell is a cell, operating on a secondary carrier, which may be configured once radio resource control (RRC) is established and which may be used to provide additional radio resources.
Although the focus to date has been principally upon frequency division duplex (FDD) networks, time division duplex (TDD) networks that support carrier aggregation must also be considered. Indeed, in a TDD network, the primary cell and the secondary cells may have respective TDD uplink (UL)/downlink (DL) subframe configurations. In Long Term Evolution (LTE) release 10, mobile terminals that support TDD signaling are required to operate in accordance with a TDD UL/DL subframe configuration that is aligned and consistent across the primary and secondary cells that are to be aggregated. Additionally, LTE release 10 required that common discontinuous reception (DRX) parameters be utilized for each of the primary and secondary cells such that the active time and the DRX pattern would be the same for each of the aggregated cells.
For mobile terminals configured in accordance with LTE release 11, however, the primary and secondary cells are permitted to have different TDD UL/DL subframe configurations. These different TDD UL/DL subframe configurations may be useful in order to allow a cell to be compatible with a neighbor legacy TDD system, such as a Time Division Synchronous Code Division Multiple Access (TD-SCDMA) system, for which multiple cells having aligned TDD UL/DL subframe configurations are not available. Additionally, primary and secondary cells having different TDD UL/DL subframe configurations may provide different amounts of resources and a different DL/UL ratio for the mobile terminal. For example, the use of different TDD UL/DL subframe configurations by the primary and secondary cells in LTE release 11 may permit different ones of the cells to provide different coverage by, for example, enabling more UL subframes in a lower frequency cell to enlarge the coverage. The ability to provide different amounts of resources and a different DL/UL ratio may be of particular importance to a mobile terminal that utilizes carrier aggregation since carrier aggregation is oftentimes utilized in an instance in which the mobile terminal has a relatively large amount of data to transmit, thereby increasing the importance of defining the TDD UL/DL subframe configurations of the primary and secondary cells so as to provide a suitable DL/UL ratio and to otherwise efficiently utilize the communication resources.
By allowing the primary and secondary cells to have different TDD UL/DL subframe configurations, however, the different TDD UL/DL subframe configurations of the primary and secondary cells may have overlapped subframes in some instances, such as by one of the cells having a DL subframe at a specific instance in time while another cell has an UL subframe. An example of the TDD UL/DL subframe configuration of a primary cell and a secondary cell is shown in FIG. 1. In this example, subframe 3 is overlapping in that subframe 3 of the primary cell is a UL subframe, while subframe 3 of the secondary cell is a DL subframe.
The mobile terminal may be configured to process one of the overlapping subframes with the other of the overlapping subframes being blocked or otherwise ignored. In order to avoid missing any transmission and reception opportunities, a mobile terminal may be alternatively configured to provide for simultaneous reception and transmission so as to accommodate overlapping subframes.
The TDD UL/DL subframe configuration of a cell normally defines its hybrid automatic repeat request (HARQ) timing and reception. However, in an instance in which the mobile terminal is configured for simultaneous transmission and reception, the HARQ timing and procedure may not directly follow the HARQ timing and procedure otherwise defined for the TDD UL/DL configuration of the respective cells. In this regard, in LTE release 10, the HARQ UL feedback is always supported by the primary cell and the HARQ DL feedback always has the same location as DL assignment. Thus, if the HARQ timing and HARQ process number for each TDDUL/DL subframe configuration of the primary and secondary cells were directly utilized, the overlapping subframes may block some of the desired feedback.
With reference to FIG. 1 in regards to DL HARQ, for example, the primary and secondary cells have TDD UL/DL subframe configurations #1 and #0, respectively. In an instance in which the HARQ timing for TDD UL/DL subframe configurations #1 and #0 were followed, the DL HARQ, that is, the uplink feedback timing, for subframe #0 for the secondary cell would be provided in subframe #4 of the primary cell, since the uplink feedback is always supported by the primary cell. However, subframe #4 of the primary cell is not a UL subframe, but is, instead, a DL subframe. Thus, the UL feedback for subframe #0 of the secondary cell will be blocked and will not be provided by subframe #4 of the primary cell.
In regards to the UL HARQ, which includes the UL grant and the DL feedback timing, a mobile terminal may also experience issues as a result of overlapping subframes in an instance in which cross-scheduling is enabled for the mobile terminal. For example, in an instance in which a secondary cell is cross-scheduled by the primary cell, DL feedback which is carried on the physical HARQ indicator channel (PHICH) on the primary cell will be blocked in an instance in which the subframes providing the DL feedback overlap. As shown in FIG. 2, a secondary cell having TDD UL/DL subframe configuration #1 may provide DL feedback for subframe #3 of the secondary cell on subframe #9. Since the DL feedback has the same location as the DL assignment, the DL feedback will be supported by the primary cell. However, subframe #9 of the primary cell is not a DL subframe in the example of FIG. 2, but is, instead, a UL subframe. As such, the DL feedback will be blocked by the overlapping subframes in this example. A comparable issue with respect to blocking or missing of the UL grant may also be brought about by the overlapping subframes.